Most folks say a 4L80e will take 500/500 HP/torque.
Gear Vendors is good but when I bought one and tried to fit it I found that you needed to cut the main reinforcing crossmember through the center of the unibody had to be cut and a plate rewelded to replace it. I was unwilling to do that for two reasons. First, my car is a Super Duty and that kind of major surgery was unthinkable. Second, I questioned whether I was compromising the structural integrity of the car doing it.